The standalone Long-Term Evolution (LTE) in unlicensed spectrum (LTE-U) forum and 3rd Generation Partnership Project (3GPP) Release 14 (Rel-14) work item on Uplink Licensed-Assisted Access (LAA) intends to allow LTE User Equipments (UEs) to transmit on the uplink in the unlicensed 5 GHz or license-shared 3.5 GHz radio spectrum. For standalone LTE-U, initial random access and subsequent UL transmissions take place entirely on unlicensed spectrum. Regulatory requirements may prohibit transmissions in the unlicensed spectrum without prior channel sensing.
Because the unlicensed spectrum is typically shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) method may be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy.
Today, unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard, also known as Wi-Fi.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform spread (DFT-spread) OFDM (also referred to as single-carrier FDMA [SC-FDMA]) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in Figure (FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing (15 kHz) as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe comprises 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs when including the cyclic prefix.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting from 0 at one end of the system bandwidth.
Up to now, the spectrum used by LTE is dedicated to LTE. This has the benefit of allowing LTE to avoid complications from sharing the spectrum and to achieve commensurate gains in spectrum efficiency. However, the spectrum allocated to LTE is limited and cannot meet the ever increasing demand for larger throughput from applications/services. Consequently, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
Unlicensed spectrum can, by definition, be simultaneously used/shared by multiple different technologies. Therefore, LTE should consider coexistence with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects the channel is occupied.
Furthermore, one way to utilize unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. For example, as shown in FIG. 3, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this description, a secondary cell in unlicensed spectrum is referred to as a licensed-assisted access secondary cell (LAA SCell).
A new industry forum has been initiated on extending LTE to operate entirely on unlicensed spectrum in a standalone mode, which is referred to as “MuLTEfire”. In MuLTEfire there is no licensed carrier for essential control signals transmissions and control channels. Accordingly, all transmission occurs on unlicensed spectrum with no guaranteed channel access availability, yet it must also fulfill the regulatory requirements on the unlicensed spectrum.